Such tabs have been used on some aircraft for providing power to assist the movement of a control surface. For example, as shown in U.S. Pat. No. 3,363,862, an elevator is disclosed having a control tab pivotally mounted on the aft end of the elevator. The tab is mechanically linked to the pilot's controls such that the pilot mechanically moves the tab. Deflection of the tab results in aerodynamic forces being exerted on the tab, and these forces in turn cause the elevator to pivot in the opposite direction until the elevator and tab system reaches an equilibrium condition.
More recently, aircraft control surfaces have generally been actuated by hydraulic devices. In some hydraulically powered control systems, geared tabs have been used in order to reduce the hinge moments of trailing edge control surfaces. In a geared tab arrangement, a tab is pivotally connected to the trailing edge of the control surface and the tab is mechanically linked to aircraft structure such that pivotal movement of the control surface in one direction causes a pivotal movement of the tab in the opposite direction. The aerodynamic moment about the control surface hinge axis is reduced by the moment exerted by the forces on the tab. A drawback of a geared tab arrangement is that the gearing ratio between the control surface and the tab is fixed and thus cannot be optimized for all flight conditions.
This drawback was remedied in part by the development of a "reversing" geared tab for an elevator as described in U.S. Pat. No. 4,431,149 issued to Brislawn et al. The reversing geared tab described in the '149 patent provides tab deflections that are inversely proportional to (i.e., in the opposite direction to) the elevator deflection for all trailing-edge-down (TED) elevator deflections, and for trailing-edge-up (TEU) elevator deflections from zero to about 13.degree.. This inversely proportional tab deflection reduces the elevator hinge moments and over-sensitivity for elevator deflections typical of high-speed flight. However, at 13.degree. of elevator deflection, the tab gearing ratio changes sign so that the tab starts to move in the same direction as the elevator for TEU elevator deflections greater than 13.degree., resulting in the tab returning to a faired position at the maximum TEU elevator deflection of 35.degree.. This provides normal elevator effectiveness at maximum TEU elevator deflection (i.e., no tab losses) as required for low-speed flight.
While the reversing geared tab of the '149 patent is an improvement over a non-reversing geared tab, the fixed schedule of gearing ratio provided by the reversing geared tab system is not capable of providing optimum tab hinge moment balancing over the full range of operating conditions. Furthermore, the geared tab of the '149 patent does not function as a "leading tab" wherein, for example for takeoff rotation, the tab deflection is in the same direction as and exceeds that of the elevator so as to provide increased elevator control power.
Next-generation Boeing 737 aircraft employ a reversing elevator tab that functions both as a balancing tab to reduce hinge moments in high-speed cruise operations, and as a leading tab to provide increased elevator control power for takeoff rotation. The gearing ratios in both the balancing mode and the leading mode are fixed. If hydraulic power is lost at any time, the tab automatically reverts to the balancing mode in order to minimize hydraulic power requirements.
The present invention was developed at least in part to address the unique control needs presented by the Boeing Blended-Wing-Body (BWB) aircraft that is under development by The Boeing Company. The BWB aircraft is essentially a large flying wing. To provide longitudinal stability of the aircraft, the wing requires large trailing edge control surfaces across the entire wing span, and the control surfaces must be capable of deflecting at substantially higher rates than those of moreconventional aircraft. Accordingly, the BWB aircraft presents some technical challenges in terms of providing sufficient hydraulic power to drive the control surfaces in high-speed cruise operations. Additionally, in low-speed operations, especially at takeoff, increased pitch control power may be needed beyond that which the elevator control surfaces are able to provide. A further challenge is to assure that the control surfaces can be driven as needed to provide a minimally acceptable amount of control power during a hydraulic power failure such as when all of the engines fail. The present invention seeks to provide a solution to all of these challenges.